1. Field of the Invention
This invention relates to a crystalline random propylene copolymer composition which can give a polypropylene composite laminate having improved heat sealing property by laminating it to the surface of a crystalline polypropylene substrate, and to a laminated composite so obtained.
2. Description of the Prior Art
Crystalline polypropylene films have found extensive use in the field of packaging, especially food packaging, because of their superiority in mechanical properties such as tensile strength, rigidity, surface hardness and impact strength, optical properties such as gloss and transparency, and food hygiene such as the freedom from toxicity and odor. They, however, have the defect that temperatures at which a single layer of polypropylene film can be heat-sealed are high and a proper range of these temperatures is narrow.
In an attempt to improve the heat sealing property of polypropylene films, some methods have already been proposed in which a low-melting resin is laminated to one or both surfaces of a crystalline polypropylene film.
For example, Japanese Laid-Open Patent Publication No. 65552/1980 discloses a process in which a propylene random copolymer composition composed of a random ethylene/propylene copolymer comprising propylene as a main component and a random propylene/alpha-olefin copolymer comprising propylene as a main component is laminated to a crystalline polypropylene film.
Japanese Laid-Open Patent Publication No. 91665/1980 discloses a process in which a random propylene copolymer composed of an ethylene/propylene random copolymer comprising propylene as a main component and a random 1-butene/ethylene copolymer comprising 1-butene as a main component is laminated to a crystalline polypropylene film.
Japanese Laid-Open Patent Publication No. 106585/1979 discloses a process in which a random propylene copolymer composition composed of a random ethylene/propylene copolymer comprising propylene as a main component, a copolymer of 1-butene and an ethylenically unsaturated monomer comprising 1-butene as a main component and a low-molecular-weight thermoplastic resin is laminated to a crystalline polypropylene film.
U.S. Pat. No. 4,230,767 claiming Convention priority from the above-cited Japanese Laid-Open Patent Publications Nos. 91665/1980 (Japanese Patent Application No. 165137/1978) and 106585/1979 (Japanese Patent Application No. 13932/1978) discloses a random propylene copolymer composition which can embrace a composition composed of a random ethylene/propylene copolymer comprising propylene as a main component and a 1/butene-propylene copolymer containing 1-butene as a main component and includes an embodiment in which such a propylene random copolymer is laminated to a crystalline polypropylene film.